1. Field of the Invention
The present invention relates to a surface treating for micromachining and surface treatment method, and more specifically to a surface etching material and a surface etching method using this material that are extremely effective when used for cleaning in a wet process for microscopic processing of oxide at the time of manufacturing a semiconductor element, and for cleaning a surface or a semiconductor element surface that has been microscopically processed.
2. Description of the Related Art
In wet processes for semiconductor integrated circuits, there are increasing demands for cleaning, precision and increase in speed for cleaning, etching and patterning of a wafer surface and a micromachined surface accompanying the development of integrated circuits. A mixed solution of HF and ammonium fluoride (NH4F) (buffered hydrofluoric acid), along with a micromachining surface treatment material essential to this process, is used for the purpose of cleaning, etching and patterning, but for ultra high integration in the submicron range there is a need to improve performance and functionality.
Particularly, with the advancement of high integration, it has become possible to make contact holes as small as 0.25 xcexcm, or 0.18 xcexcm. The number of wires and the number of manufacturing steps have also increased. Also with respect to interlayer insulation film, as well as conventional CVD films, organic TEOS films are also being used.
When this TEOS film is used, a heat treatment process is carried out at a temperature of 500xc2x0 C. to 800xc2x0 C., but with the advancement of microscopic techniques, when making a 1 Gbit DRAM in which the wiring width and the material are limited, it is necessary for the heat treatment temperature to be kept to 500xc2x0 C. at the edge of a transistor and less than 250 to 300xc2x0 C. in the vicinity of the interlayer insulation film. If the heat treatment temperature is lowered, a CVD oxidation film or a TEOS film etc. have an extremely slow etching rate compared to HF chemical solution.
Also, when contact holes are processed by dry etching, for example, when dry etching is carried out with, for example, a diameter of 0.18 xcexcm and a depth of 1 to 2 xcexcm, precedence is given to dimensional accuracy and etching shape, which means that currently etching is performed with no consideration given to damage of the bottom of the contact holes. As a result, a substrate or a polysilicon film at the bottom of the holes is subjected to damage, and in particular about 20 nm of a substrate surface approaches amorphous silicon. Also, in the vicinity of a lower 30 nm of the substrate, is doped by ion injection of CF, being a dry etching gas.
In order to eliminate this damage, or when removing resist, a cleaning stage is becoming increasingly important, and, for example, RCA cleaning is becoming absolutely essential, but if this type of cleaning is carried out a natural oxidation film is formed, and a method of removing this natural oxidation film poses a problem.
Conventionally, removal of this type of natural oxidation film mainly utilized rare HF or buffered hydrofluoric acid (BHF). In the case of BHF it was possible to use a chemical solution of, for example, 100:1 BHF (40% NH4F:50% HF=100:1) to etch a thermal oxidation film at 25xc2x0 C., at about 10 nm/min.
However, when removing a natural oxidation film at the bottom of contact holes opening onto an oxidation film (a TEOS film) heat treated at a low temperature, since the etching rate of the TEOS film of a side wall having a low heat treatment temperature is extremely slow, it is possible that holes of 0.25 xcexcm diameter will be widened to 0.5 xcexcm after cleaning. Specifically, with rare HF or 100:1 BHF, removal of a natural oxidation film is difficult from a practical standpoint without widening the hole diameter. For this reason, conventionally, patterning was performed for a 0.2 xcexcm hole and a margin was designed in to allow for widening due to etching. However, with the continuing progress with respect to fine patterning, with 0.18 xcexcm and 0.15 xcexcm design rules even that margin is not permissible.
In the removal of a natural oxidation film formed at the bottom of contact holes, it is necessary to make a selective etching ratio of the natural oxidation film etching rate to the side wall (CVD film, TEOS film etc.) etching rate large. That is, it is actually desirable to remove only the natural oxidation film. However, from a practical standpoint, the natural oxidation film etching rate is high compared to that of the CVD film or the TEOS film, and when also taking mass production processes into consideration, the time required to remove the natural oxidation film is not necessarily as short as possible. Therefore, when a wafer is processed using a wet station, in the case of an 8 inch wafer, batch processing using a cassette containing 25 wafers at one time is most common. The problem in this situation is the time to convey the wafer cassette from an etching chamber to a purified water chamber. Accordingly, the time for removing the natural oxidation film is desirably from 30 seconds to a minute, which can be mechanically controlled Specifically, if the time taken to remove the natural oxidation film is about 10 seconds, the time that the wafer is in contact with processing fluid eventually becomes about one minute when taking into consideration over-etching time for complete removal and conveyance time. As a result, the side wall sections of the contact holes are etched for the same amount of time.
Accordingly, the object of the present invention is to provide a surface treating for micromachining and surface treatment method that fixes etching time to about one minute, slows an etching rate of a natural oxidation film as much as possible in this range of etching time, and enables a selection ratio to that for a CVD oxidation film or a TEOS film to be made small.
The micromachining surface treatment material of the present invention contains less than 0:1% hydrofluoric acid, and more than 40% by weight but less than or equal to 47% by weight of ammonium fluoride.
The micromachining surface treatment material of the present invention is manufactured by dissolving ammonia gas in a hydrofluoric acid solution.
Further, the micromachining surface treatment material of the present invention contains a surfactant at 0.0001 to 1% by weight.
The present invention also provides a surface processing method for removing a natural oxidation layer inside a contact hole using the above described micromachining surface treatment material.